Hi Leif,
Sounds like it has to do with surface tension.
SnPb and SAC have significant surface tension differences, and getting 
differential cooling also causes differences in surface tension.
One way to reduce the effect of surface tension (thus, reducing tombstoning 
and component cracking). is to make the pads narrower than the components.
If you swim up the component, you get an increase in SJ reliability as a 
bonus.

Werner
Future workshops:
Reliability Issues with Lead-Free Soldering Processes, June 17, London
Interconnect Failures and Design for Reliability for PCB PTHs, June 17, 
London
Pb-Free Soldering Processes—Survival, Quality, Reliability, August 18, 
Orlando
Reliability Issues with Lead-Free Soldering Processes, September 22, 
Schaumburg
Failure Mode and Root Cause Analyses Reliability (Fatigue, Brittle Fracture, 
ENIG), September 22, Schaumburg



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